I believe people need to be educated in order to be able to make good choices about the foods they eat and drink. The problem is, as Hv pointed out, is that corporations and the food industry bodies lie about what is in our foods and how those ingredients can effect our health. It is an absolute mine field of missinformation, on one hand we are told that artificial sweetners are good for us because we won't be getting the calories that we would from sugar, then we find out this is a lie (we know this is a lie because we actively seek out the science, most people don't do this, they believe what the corporations tell them). People are told that fluoride in the water is great for them, vaccines containing mercury are good for your health and so on and so forth.
We live in a society that likes to blame the victims, and people who are overweight are the victims of coporate lies.
Having said that, there is certainly an amount of personal responsibility that needs to be acknowledged in the case of people who are overweight or obese. There is enough information around today that I don't believe anyone in the western world is unaware of the fact that if you eat a high fat , high Sugar diet , do not get adequate exercise, you will get fatter, it is a known fact. I believe that it is a combination of buying the corporate lies, and making bad health choices, that has led to the epidemic of obesity in our society.
There is just one other point that I would like to make in this post, that is, in my own country, we work some of the longest hours in the world, this imbalance in our lives causes a lack of time to pursue activities that are beneficial to our health. It also adds to the already large amounts of stress in our lives and this also adds to the ill health of our country.